In electrophotographic copying machines according to the Carlson's system, image formation is accomplished by charging the surface of a photoreceptor followed by imagewise exposure to light to form an electrostatic latent image, developing the latent image with a toner, and transferring and fixing the toner image onto paper, etc. while destaticizing and cleaning the photoreceptor to remove the residual toner and residual charge. The electrophotographic photoreceptor is repeatedly used for a long term.
The electrophotographic photoreceptor is therefore required to have not only satisfactory electrophotographic characteristics, such as charging characteristics, sensitivity and dark decay, but also physical properties sufficient for withstanding long-term use, such as printing durability, abrasion resistance, and moisture resistance, as well as resistance to ozone generated at the time of corona discharging and to ultraviolet light on exposure (i.e., resistance to the environment).
Recent studies have been directed to organic photoreceptors in which a charge generating function and a charge transporting function are separately performed by different materials in pursuit of a highly sensitive and highly durable photoreceptor. In designing such a so-called separate function type electrophotographic photoreceptor, the material bearing the respective function can be selected from a broad range, making it relatively easy to provide a photoreceptor with characteristics as desired. However, the photoreceptor of this type cannot get rid of the problem of deterioration in electrophotographic characteristics and resistance to environment on repeated use. That is, the photoreceptor suffers from a reduction in chargeability, an increase in residual potential, an increase in potential in the white background area (exposed area), a reduction in photosensitivity, and the like. Since charging by a corona discharge is attended by generation of active substances such as ozone, the photoreceptor is affected by these active substances, causing image quality deterioration, such as blurs.
In order to overcome these problems, various proposals have been made to date. For example, JP-A-2-293853 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses an electrophotographic photoreceptor using two kinds of charge transporting materials, one of which has a greater ionization potential than that of a charge generating material, the other having a smaller ionization potential than that of a charge generating material. However, the proposed photoreceptor is not always free from the above-mentioned problems on repeated use.